We'll see how that revelation impacts both Sherlock and his partner, Joan Watson, in tonight's two-hour finale. Here are five things to expect from the supersized episode, courtesy of creator and executive producer
Rob Doherty, who participated in a conference call with journalists to discuss the ending.

Moriarty has been cast (which means he's probably going to appear in the finale)

"It is no mean feat. Of course I can't say much beyond that," Doherty says of the casting process. "[He's] a vastly important and iconic character and certainly someone we've been building towards all season. So a lot of thought and time went into it. The truth is I had some very specific thoughts and was lucky enough to latch onto a first choice." Doherty later added the mystery actor is "Somebody who's work I was very familiar with. Someone I really was excited to have an opportunity to work with [and] just made perfect sense for us."

Moriarty's motivations will also be revealed and explored

"For lack of a better analogy [Moriarty is] the other side of Sherlock's coin -- somebody who's quite like him but has been drawn in a very different direction," Doherty explains. "I always felt like Sherlock and Moriarty are the only two of their kind on the planet. And so when one realized the other existed his curiosity was peaked.

"In this case it's more Moriarty was the first to become aware of Sherlock. And so it's interesting. If you spend your life thinking you're the only person on the planet with these gifts, with these abilities who sees the world this kind of way, it's fascinating to think maybe there's one other person out there. Even though Moriarty has tortured Sherlock, I think what we're going to see is there is also a kind of mutual respect and mutual interest."

"We really wrote to Moriarty's interest in Sherlock. Moriarty's not just a frustrated mastermind who wants to break the bad guy. [He's] more interesting than that. Does that make Moriarty more likable? No, no. He's still doing bad things."

If Joan is going to bond with Irene, it will have to happen later

"At least for the first season we're only going to get a couple of episodes where Irene and Joan will get to share some significant screen time," Doherty says. "I'd say as we move into next week's episode, Irene has been through quite an ordeal. In a perfect world, Joan at her best would get to meet Irene at her best. But Irene is somebody who needs some help and a little time. What you'll see is a Joan who's trying to help Sherlock help Irene."

Irene's arrival is intended to rock Sherlock's world... and it will

"It's hard -- at least in the world of our show -- to find a bigger trigger than Irene, dead or alive," Doherty explains. "The fact that she has turned up alive has thrown Sherlock for a loop. He's been tricked. He's been deceived. I think in Moriarty's eyes you could say [Sherlock] bottomed out over nothing. He spiraled out of control because he thought Irene was dead when in fact she was quite alive and being kept by Moriarty.

"So at least at the moment the joke's on Sherlock -- Joan has to be concerned about that. I think in addition to trying to help Sherlock situate Irene and help her recover from her ordeal, she also has to keep an eye on Sherlock and make sure he's able to stay on the straight and narrow."

A certain amount of resolution was necessary to properly set-up Season 2

"For a good long while we anticipated a cliffhanger [ending to the season]," Doherty admits. "When it looked more likely that we would get to go to London for our [Season 2] premiere, we didn't want to drag any of this year's business into a second season, because I feel like London can be its own show. I didn't want to have to connect it back to the first season. The notion of going to London came up early enough for us to write what I feel is a nice last chapter for this season. We will be tying things up and starting with something of a clean slate coming into year two.

"These last few episodes were a blast to write and I love writing to the Moriarty of it all. But it's so heavy for Sherlock. It's when you write about Moriarty and Irene and his drug habit, it's appropriately dark. It's true to the character and it's important stuff. But again, it doesn't make for much fun in the U.K."